Document fail-fast error handling philosophy in FanOut

[Copilot]

Addresses review feedback requesting no panic recovery in FanOut's Run() method. The implementation already follows fail-fast principles; this PR makes the design decision explicit.

Changes

• Package documentation: Added "Error Handling" section documenting fail-fast philosophy
• Run() documentation: Clarified panic behavior for multiple calls and absence of recovery
• Build artifacts: Added coverage.txt to .gitignore

Behavior

// Run() panics on programmer error (multiple calls)
fo.Run(ctx, input)
fo.Run(ctx, input) // panics: "fanout: Run() called multiple times"

// No recovery in dispatcher - any panic propagates and crashes
// This prevents silent corruption and ensures correct behavior

The dispatcher goroutine intentionally lacks panic recovery to ensure failures are visible and immediate rather than masked.

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